Cast resin commutating rheostat



United States Patent CAST RESIN COMMUTATING RHEOSTAT Application September 30, 1955, Serial No. 537,858

Claims. (Cl. 201-48) This invention relates to a commutating rheostat and,

more particularly, to a commutating rheostat for a governor.

In the past, the use of commutating rheostatsand voltage dividers where the resistance wire is exposed to the atmosphere has resulted in deterioration of the re sistance wire because of oxidation under severe high temperature usage. Attempts to encase the resistance wire in a resin has allowed the use of less expensive resistance wire and has resulted in better heat characteristics and a longer life for these devices. However, past methods of making such rheostats were expensive and it was found that many commutating rheostats using improper insulation developed a high resistance film on the contact surface resulting from the combined deterioration of the contact surface, the resin used and the atmosphere. This film often had an impedance greater than the impedance of a turn of the resistance wire which would result in considerable hunting by an automatically controlled brush arm for the right voltage potential.

Therefore, an object of our invention is to provide a low cost, simple and reliable commutating rheostat.

In carrying outour invention in one form, a resistance wire is wound on a ceramic core. A silver ring or washer is soldered to the turns of the resistance wire and then slotted between the wire connections to produce an annular commutating surface having integral commutating segments individually supported by resistance wires. Thereafter the assembly is molded in an epoxy resin to provide a completely insulated resistance wire commutating rheostat.

Further objects and advantages will become apparent and our invention will be better understood from the following detailed description and the accompanying drawings.- The features of novelty will be pointed out with particularity in the claims annexed to and forming a part of this specification.

Fig. 1 shows the toroidal core with resistance wire and the unslotted commutating ring in place;

Fig. 2 shows a detail perspective view of a portion of the core and the slotted commutating surface;

Fig. 3 is a sectional view of the assembled commutator in the mold with the resin; and

Fig. 4 is a perspective view of the completed commutation rheo-stat, I

Referring nowto the drawings, we have shown an insulated toroidal or thick hollow cylindrical core support member 1 with a resistance wire 2 wound thereon. We prefer that the core 1 be thick enough to provide sufficient dielectric strength and mechanically strong enough to properly support the wire 2 and made of a material which will not be affected thermally or chemically by the molding operation described below. We have found that ceramic cores are very satisfactory. One end surface of the core 1 should be of a suitable shape, preferably flat, to provide an annular plane commutating surface 4. In order to facilitate the winding of the resistance wire ill 2,866,051 Patented Dec. 23, 1958 2 2 on the core 1, it is preferred that the core 1 be pro vided with grooves 3 in the chamfers thereof (Fig. 2) at a uniform spacing in which the wire is laid.

The resistance wire 2 is wound or coiled axially on the ceramic core 1 so as to have the turns or loops each lying in a plane substantially parallel to the core axis in the desired spacing determined by the index grooves 3 in the core, to provide the desired impedance characteristics. The portions of the turns of the wire 2 extending across and lying on the plane end surface 4 of the core 1, including a portion of each end of the wire, are then machined or compressed on the surface 4 to provide slightly raised portions on the core 1 of a uniform height, i. e., with the outer surfaces of the pertions lying in a common plane.

In order to provide a low resistance commutating surface with a minimum of labor, a solid silver ring 5 is then integrally secured at 6, as by soldering or brazing, to each of the cross portions of the wire on the surface 4 as shown in Figure l. The assembly of the core l, the Wire 2 and the ring 5 is then placed in an indexing head of a lathe or cutter and the silver ring 5 is slotted between each turn of the resistance wire 2, i. e., between the integral connections, leaving individual silver commutating bars 5 in a common plane connected to and supported each by a turn of the resistance wire 2 to provide an annular commutating surface. This method requires less labor and results in fewer rejects than past methods where the bars were precut and placed on the wire 2 before or after winding the wire on the core 1.

This assembly is then secured in a tapered greased mold 8 of suitable material such as aluminum with conductor lead bolts 9, 10 and 11 connected respectively by threading or other means to one enlarged end bar 13 of the commutator; an annular slip ring 14 proiding a voltage center tap contact surface, and the other enlarged end bar 15 of the commutator. It should be noted that the enlarged end bars 13 and 15 are cut from the ring 5 at the same time as the bars 5 and are supported by end turns of the resistance wire 2. In some applications it may be preferable to have the bolts 9 and 11 pass through perforations (not shown) in the core 1.

A base plate 17 having an attached hollow cylindrical bearing shaft 18 is then placed over the open end of the mold assembly with the shaft 18 positioned axially therein. The base plate 17 is preferably of a strong material and a good heat conductor such as aluminum and is provided with apertures 19 surrounding but not contacting the terminal lead bolts 9, 10 and 11.

After the base plate 17 is suitably clamped in place by means not shown, a liquid epoxy casting resin 20 is injected through openings (not shown) in the base plate 17 into the mold 8. In order that the casting resin will fillin all available space including the slotted portions between the commutator bars 5, it may be desirable to vibrate, place in a vacuum, or treat with other suitable means the full mold 8 to remove any air bubbles entrapped therein. It has been found that these bubbles, if allowed to remain in the resin, tend to reduce the heat conductivity and dielectric strength of the resin. After the resin 20 has been properly cured, the core assembly is secured by the resin to the coaxial collector ring 14 and the base plate 17 to form an integral unit. This unit is then removed from the mold 8.

We have found that the mold 8 should be provided with concentric recesses 22, 23 and 24 to receive the silver commutator segments 5', 13 and 15, the contact ring 14' and the axial bearing shaft 18, respectively. With this construction the cooperating movable parts are coaxially aligned and the contact surfaces are offset from the solid resin so that they may be more easily cleaned to provide low resistance contact surfaces. It should be noted that the resin partially surrounding the commutator bars 5', 13 and 15 adds to the support thereof resulting in a vibration resistant assembly.

In order to clean the contact surfaces and to undercut slightly the epoxy resin between the silver commutating bars 5', 13 and 15, a vapor blasting operation with very fine aluminum oxide in water or a sand blasting operation is used on the commutating surface of the assembly. Preferably vapor blasting is used to highly polish the commutating surface. A nozzle suitable for conducting the vapor to the molded member is shown in Fig. 4. We prefer that any vapor blasting does not cut deeply enough to expose any of the resistance wire 2.

We have found that the epoxy resin 20 may be filled with any suitable thermally conductive fillers such as powdered aluminum or carbon to improve the heat conductivity and/ or radiation and thus more effectively cool the resistance wire 2 and increase the current rating of the rheostat. The epoxy resins we prefer are of the ethoxyline class as defined in Letters Patent 2,707,177, entitled Ethoxyline-Triallyl Cyanurate Resinous Compositions, issued April 26, 1955, to R. A. Skiff and F. W. Finholt and assigned to the assignee of the present application. This class of resins provides outstanding adhesion to a wide variety of materials and has little or no volatile or toxic material given off during curing.

This method results in a stable and airtight and, therefore, corrosion-proof commutating rheostat assembly wherein the resistance parts are completely encased. We have found that a solid casing materially increases the power rating of a given rheostat by as much as 75 percent. The low shrinkage during curing of the particular resin 20 used will prevent stresses in the resin without requiring the curing to be carried out under pressure. The resin, after being cured, is hard but not brittle so that abrasion will not wear through the resin and shock loads will not crack the resin. from fouling the space between the turns and the rheostat maybe used in applications which would normally require special dirt shields or covers. The dielectric strength of the resin is sufficient to prevent any shunting therethrough between the resistance wire turns on the grooved core 1, even if a large percentage of electrically conductive filler is used. Moreover, the resin is sufficiently heat resistant or refractory to protect the resistance wire from oxidation when the wire is heated to high temperatures. does not result in the forming of a film on the commutating surfaces sufficient to cause hunting of an automatically controlled brush arm.

After the resin encased portion of the rheostat is removed from the mold and vapor blasted, the knob assembly (Fig. 4) is suitably secured within the bearing shaft 18. The insulated knob 26 has secured thereto a rotary insulated contact brush arm 27 which shunts the center tap contact ring 14- to various commutator bars 5' or the enlarged end bars 13 and 15. Of course, in some applications we have found it desirable to connect a center tap terminal electrically to the bearing shaft 18, thus eliminating the center tap contact ring 14 and the lead bolt 10. However, in heavy power applications, we prefer to use the commutating rheostat shown in Fig. 4.

While we have shown and described the particular embodiments of our invention, modifications thereof will occur to those skilled in the art, and we intend by the appended claims to cover all such modifications that do not depart from the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

. 1. A rheostat comprising a ceramic core having a resistance .wire wound thereon, a plurality of low im- Thus the resin prevents dirt We have found also that the use of this resin pedance commutator bars each being soldered to and supported on an outer surface of one separate turn of said wire to provide an annular communicating surface, terminals connected respectively to the ends of said wire and connectable to a voltage source, an electric contact mechanism for contacting one of said commutator bars to tap a voltage therefrom, and an epoxy resin coating covering said core and said wire to insulate said wire entirely from corrosion and from electric shunting between individual turns.

7., A rheostat comprising a toroidal ceramic core provided with spaced indexing grooves, a resistance wire wound axially thereon and lying in said grooves, a plurality of commutator bars integrally secured each on an outer surface of a separate turn of said wire to provide an annular cornmutating surface, a pair of electric terminals each connected to an end one of said bars and connectable to a voltage source, an annular slip ring, a third terminal connected to said slip ring, a contact arm for selectively connecting one of said commutator bars to said slip ring, a base plate, said base plate being provided with apertures to allow the passage of said terminals therethrough, and an epoxy resin covering said core and said wire to insulate said wire entirely from corrosion and from electric shunting between individual turns and to connect mechanically said wires, said slip ring and said terminals to form an integral unit, said coating being partially undercut between said commutator bars, and said terminals being exposed to provide electric connections thereto.

3. A rheostat comprising a core member of an insulation material, resistance material having a plane annular surface provided with slightly raised portions supported on said core member, integral commutator bars connected to and supported on the surface of each of said raised portions, terminal means electrically connected to respective ends of said resistance material for providing a voltage thereacross, a brush adapted to selectively contact said bars individually and an epoxy resin coating covering said resistance material including said raised portions to protect the same from corrosion and electric shunting.

4. A rheostat comprising a core member of an insulation material, resistance wire wound on said core memher to have a plane annular surface of a uniform height, integral commutator bars each connected to and supported on one of said wires on said annular surface, terminal means electrically connected to respective ends of said resistance wire to facilitate providing a voltage thereacross, an epoxy resin coating covering said resistance wire including said annular surface to protect the same from corrosion and electric shunting, a brush member adapted to selectively contact said bars individually, a terminal connected to said brush member, and means drivingly connected to said brush member for controlling the position thereof and thereby controlling the voltage of said brush member terminal.

5. A rheostat comprising a core of an insulation material having therein spaced indexing grooves, resistance wire wound on said core member and spaced by said grooves to have a plane annular surface of a uniform height, integral commutator bars each soldered on top of and supported by adjacent turns of said wire on said annular surface, terminal means electrically connected to respective ends of said resistance wire to facilitate providing a voltage thereacross, an epoxy resin coating covering said core and said resistance wire including said annular surface thereof to insulate said wire entirely from corrosion and electric shunting, said coating being in contact with said bars and said terminal means and said terminal for mechanically strengthening the entire rheostat and particularly the support of said bars, a contact arm adapted to selectively contact said bars individually, a terminal connected to said contact arm, and means drivingly connected to said contactor for control- References Cited in the file of this patent UNITED STATES PATENTS Saucy May 18, 1920 Siegel Feb. 27, 1934 6 Swartz et al. Sept. 4, 1934 Smith Sept. 7, 1948 Bradley Mar. 14, 1950 Ekstein Feb. 9, 1954 Polard Dec. 14, 1954 Frisbie et a1 Aug. 30, 1955 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,866,051 December 23, 19:38

Russell .A... Skiff et It is hereby certified that error appears in the printed specification of the above nu mbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line for "commcmioating" read conmutatirzg e Signed and. sealed this 6th of SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,866,051 December 23, 1958 Column 4, line 3, for "comma-rimming" read w conmumtizlg Signed and sealed this 61311 of 19590 SEAL) Attest:

Commissioner of Patents 

